Generally, electromagnetic radiation detector devices comprise a matrix of individual radiation detector elements. Such radiation detector devices, also called “imagers”, further comprise a reading circuit able to receive an electrical signal from each radiation detector element. The imager is able to reconstitute a two-dimensional image from electrical signals provided by each detector element.
The radiation detector elements used are frequently made from semi-conductive materials. For example, silicon radiation detector elements of the CCD (Charge-Coupled Device) or CMOS (Complementary Metal-Oxide-Semiconductor) type are used in digital cameras capable of detecting visible electromagnetic radiation. For other radiation ranges, such as infrared radiation, other types of radiation detector elements are used, for example comprising photodiodes.
The photodiode detector elements comprise a layer capable of absorbing radiation, made from a semiconductor material. The absorbing layer comprises a p/n junction formed from the juxtaposition of a portion of p-type material and a portion of n-type material. Such a p/n junction generates an electrical field in the absorbing layer. When a photon is absorbed, it generates an electron-hole pair in the absorbing layer. The electrical field exerts a force on the electron and the hole that separates the electron-hole pair, and leads to an electric current. The electric current is next detected by a dedicated reading circuit.
Known from the article by I. M Baker, titled “Hybrid CdHgTe-Silicon infrared focal plane arrays” and which appeared in 1990 in “Fourth International Conference on Advanced Infrared Detectors and Systems”, pages 78 to 87, is a detector element of this type, comprising a semiconducting absorbing layer traversed by an electrode. The electrode is surrounded by a first portion of the absorbing layer having an n-type doping. A second portion of the absorbing layer has a p-type doping. The first portion is cylindrical, and forms, with the second portion, an n/p junction surrounding the electrode. Such a geometry of the detector elements is called “loop-hole”.
During the operation of a detector element, the electric current successively traverses an electrode, the first portion, the p/n junction, then the second portion before rejoining the reading circuit.
However, such detector elements often have a significant resistance to the passage of the current. For example, the detector element has a resistance of the order of 1011 to 1014 Ohm (Ω), which is added to a so-called access resistance of the order of 106Ω between the detector element and the electric contact. As a result, when an imager comprises several detector elements sharing a shared electric contact, the detector elements do not all have the same electrical resistance to the passage of the current. In particular, the electrical resistance increases with the distance between the detector element and the electric contact. This phenomenon is frequently called “depolarization”.
In order to limit depolarization, the second portion is generally electrically connected to the reading circuit by a metal layer placed on its surface. However, the metal layer generally absorbs part of the radiation to be detected. The performance of the detector element is therefore not optimized.
There is therefore a need for a radiation detector element having a better performance.